Musannaf Ibn Abi Shayba 3:5075

Ibn ‘Uyaynah narrated to us, from al-Zuhri, from Humayd ibn ‘Abd al-Rahman, from Abu Sa’id, that the Prophet (peace and blessings of Allah be upon him) saw phlegm in the qiblah of the mosque, so he scraped it off with his staff. He said: "If one of you spits, let him not spit in front of him, nor to his right, but let him spit to his left."